1. Field of the Invention
The present invention relates to a method of controlling the thrust of a screw of an injection molding apparatus of the type in which the screw is driven by a motor, and more particularly, to a screw thrust controlling method which is suitable for use in production of molded articles, in particular, thin molded articles.
2. Related Background Art
In the injection molding in which molded articles are manufactured by injecting a resin material such as a plastic into a mold, hydraulic type injection molding apparatuses have been employed. However, electrically driven injection molding apparatuses have been developed in recent years.
In this electrically driven injection molding apparatus, a screw is rotated by a motor, and control of the thrust of the screw is conducted by applying a current corresponding to a torque which is preset on the basis of the relation between the current of the motor and the torque thereof which is obtained beforehand.
However, since the relation between the current and the torque varies depending on the situation in which the motor is used, the above-described control method has a disadvantage in that an actually obtained screw thrust is altered from a desired screw thrust so that accurate thrust control is thus impossible. This results in generation of variations in the dimensions or strength of molded articles during the injection or dwell process, and makes mass production of uniform molded articles very difficult.
This problem of the conventional techniques has been solved by Japanese Patent Laid-Open No. 44417/1987.
In this technique, as shown in FIG. 14, a screw 71 is coupled through a load sensor 72 to a ball bearing screw 73 which is moved back and forth by the rotation of a nut 74. The nut 74 is rotated by a motor 78 whose rotational force is transmitted to the nut 74 on a route consisting of a driving gear 77, a transmitting shaft 76, and a driving gear 75. A comparator Y2 compares a thrust signal S1 which is taken out from the load sensor 72 with a signal S2 which represents a preset value of the screw thrust and which is output from a presetter Y1, and outputs a resultant instruction S to a motor controller Y3. The controller Y3 supplies a current i to the motor 78 to drive it and thereby makes the motor torque, i.e., the screw thrust, follow the preset value with a high degree of accuracy. Thus, an actual screw thrust is detected by a sensor for detecting the screw load during the injection and dwell processes, and the current value applied to the motor is controlled such that the detected screw thrust coincides with the preset value so as to produce uniform molded articles in which there exists no variation in the dimensions or strength during the injection or dwell process and to obtain accurately controlled back pressure during the plasticization process.
In the above-described injection molding apparatus, the molding material must be charged into a cavity at a fixed injection rate throughout the injection process so as to achieve tight packing of the molding material. In the case of production of thin molded articles, it is particularly difficult for the molding resin to be packed into every corner of the mold, and injection may be completed before the cavity is filled with the molding resin, generating sink or defective shape in the obtained molded articles. To obtain a fixed injection rate during the injection process is therefore essential, and this may be achieved by controlling the speed of the screw. However, in the above-described screw thrust control method in which the actual thrust of the screw is detected by the screw load sensor and the detected thrust is made coincided with the preset value throughout the injection process, screw speed is an unknown factor and it is therefore impossible to obtain a fixed injection rate.